Naphthalenedicarboxylic acid is prepared by, for example, oxidizing dialkylnaphthalene in the presence of cobalt, manganese and bromine. The naphthalenedicarboxylic acid prepared by the process, however, contains, as impurities, trimellitic acid and aldehydes produced as by-products during the course of the preparation or cobalt, manganese and bromine derived from a catalyst. If the impurity-containing naphthalenedicarboxylic acid (crude naphthalenedicarboxylic acid) is used as a starting material for preparing polyethylene naphthalate, the resulting polyethylene naphthalate is occasionally colored, or mold staining may take place in the molding process to decrease transparency of the molded products, resulting in lowering of product quality. In order to obtain high-quality polyethylene naphthalate, therefore, the crude naphthalenedicarboxylic acid needs to be purified before it is used as a starting material for preparing polyethylene naphthalate.
Japanese Patent Laid-Open Publication No. 110650/1989 describes a method of purifying crude naphthalenedicarboxylic acid comprising the steps of: causing impure 2,6-naphthalenedicarboxylic acid to react with ethylene glycol in an amount of at least 2 mol based on 1 mol of the 2,6-naphthalenedicarboxylic acid in the presence of catalytic amounts of tertiary amine and a titanium-containing compound to prepare bis(2-hydroxyethyl) 2,6-naphthalenedicarboxylate; crystallizing the bis(2-hydroxyethyl) 2,6-naphthalenedicarboxylate; and recovering the purified bis(2-hydroxyethyl) 2,6-naphthalenedicarboxylate.
National Publication of International Patent No. 508870/1993 describes a process for preparing purified dimethyl naphthalenedicarboxylate comprising the steps of: causing 2,6-naphthalenedicarboxylic acid to react with methanol in an appropriate reaction region to prepare a reaction mixture containing dissolved dimethyl 2,6-naphthalenedicarboxylate and monomethyl 2,6-naphthalenedicarboxylate; cooling the reaction mixture to a temperature of not higher than about 40.degree. C. to crystallize major parts of the dissolved dimethyl 2,6-naphthalenedicarboxylate and monomethyl 2,6-naphthalenedicarboxylate; fractionating the thus crystallized dimethyl 2,6-naphthalenedicarboxylate and monomethyl 2,6-naphthalenedicarboxylate from the reaction mixture solution; heating the thus fractionated dimethyl 2,6-naphthalenedicarboxylate and monomethyl 2,6-naphthalenedicarboxylate in a recrystallization solvent to a temperature high enough for dissolving at least a part of the dimethyl 2,6-naphthalenedicarboxylate and substantially all of the monomethyl 2,6-naphthalenedicarboxylate; recrystallizing the dimethyl 2,6-naphthalenedicarboxylate, which has been dissolved in the recrystallization solvent, at a temperature at which a major part of the monomethyl 2,6-naphthalenedicarboxylate is held in the recrystallization mother liquor; and fractionating the thus recrystallized dimethyl 2,6-naphthalenedicarboxylate from the recrystallization mother liquor.
Japanese Patent Laid-Open Publication No. 173100/1995 describes a process for preparing high-purity 2,6-naphthalenedicarboxylic acid comprising the steps of: dissolving coarse crystals of impurity-containing 2,6-naphthalenedicarboxylic acid in water in a supercritical or subcritical state; cooling the resulting solution at a temperature of not higher than 300.degree. C. to precipitate crystals; and separating the crystals from the mother liquor at a temperature of 100 to 300.degree. C.
In the circumstances, eagerly desired are development of a method of purifying crude naphthalenedicarboxylic acid by which the impurity content can be decreased through easier operations and development of a process for preparing polyethylene naphthalate by which polyethylene naphthalate having a lower impurity content and almost free from being colored or occurrence of mold staining can be obtained through easier operations.
The present invention has been made in view of the prior art as mentioned above, and it is an object of the invention to provide a method of purifying crude naphthalenedicarboxylic acid, by which a mixture of naphthalenedicarboxylic acid and a naphthalenedicarboxylic acid ester having a low impurity content or high-purity naphthalenedicarboxylic acid can be obtained. It is another object of the invention to provide a process for preparing polyethylene naphthalate, by which polyethylene naphthalate having a low impurity content and almost free from being colored or occurrence of mold staining can be obtained from naphthalenedicarboxylic acid containing impurities.